Telescopic, extendible crane booms of the general type described above are often used as outer booms in knuckle-boom cranes. In addition to the base boom section articulated to the first boom section of the crane, they often include more than one telescopic boom section. Thus, a common crane type comprises, in addition to the base boom section, three telescopic boom sections (conventionally termed "extensions"), each of which is operated by means of an associated hydraulic cylinder whose cylinder part is fixed in the above-mentioned holder and extends in cantilevered fashion inwards from the holder while the piston rod can be extended outwards from the holder, thus entraining the telescopic boom section located immediately outside. When all the telescopic boom sections are retracted in the base boom section and the crane boom has minimum length, all these hydraulic cylinders will be located close to each other adjacent the outer side of the base boom section. Then, if the cylinder parts of the hydraulic cylinders are not in accurate parallel alignment with each other, they may risk to come into contact with each other or with other components of the crane boom construction, thus giving rise to abrasion and wear, and sometimes even collision phenomena. It therefore is extremely vital that the cylinder part of each hydraulic cylinder be mounted in an accurately defined, predetermined position with respect to the holder.
In prior-art cranes, two alternatives to solve the above problem have been used. In one alternative, the outer side of the cylinder part has fixedly mounted thereon two diametrically opposed pins which engage in corresponding holes in side members of the holder while a bracket-like member projects from the holder a slight distance towards the free end of the cylinder part and is fixed thereto spaced from said pins, which retain the cylinder part against lateral movements while the bracket-like member retains it against movements in the vertical direction. A serious drawback of this alternative is that the pins on the cylinder part as well as the holes in the holder require extremely meticulous and, hence, costly preparations to ensure that the cylinder part be located in the desired position parallel to the main axis of the crane boom. Also the bracket member increases the overall construction costs. Moreover, every subsequent adjustment of the position of the cylinder part is impossible. If, therefore, any cylinder part, after the crane has been used for some time, is moved out of its parallel position with respect to the crane axis, the crane boom must be dismounted and the faulty components exchanged. Not only is this a costly operation but it also involves considerable downtimes, during which the crane cannot be used. According to the other alternative, the cylinder part is connected to the holder via a flanged joint comprising a flange which is mounted on the cylinder part, engages with a flat surface a corresponding flat surface on the holder and is connected thereto via a suitable number of screws. In this case too, extremely meticulous preparations are required for the flange as well as the holder, especially in conjunction with the welding of the components included in the holder. Moreover, any subsequent adjustment of the position of the cylinder part is made with the aid of washers. This is a troublesome and time-consuming work.